Electronic graphic systems in which the painting of a colour picture can be simulated by electronic means are known. One such system is described in our British Patent No. 2,089,625 and corresponding U.S. Pat. No. 4,514,818, the teachings of which are incorporated herein by reference. This system includes means by which the user can select from a range of colours, and a stylus and touch tablet combination for defining a point or a stroke on a picture being "painted". The stylus and touch tablet combination generates position signals representing the position of the stylus on the tablet which are translated into addresses in a store that stores signals (pixels) representing the colour being painted. New pixels are derived by a processing circuit in accordance with the selected colour, the distribution of a notional drawing implement selected by the user from a range of predefined implements, pressure applied to the stylus and the value of pixels previously held in the store, as described in the aforesaid patents. In this way the user can build up a picture as a sequence of signals representative thereof which are stored in the store and from there can be displayed on a TV type colour monitor.
Another system which includes additional means to enable a user to perform picture composition in addition to painting is described in our British Patent No. 2,113,950 and corresponding U.S. Pat. No. 4,602,286 the teachings of which are also incorporated herein. In this system, storage means are provided for storing two independent pictures which are combined under the users control by way of a control image or stencil held in a stencil store. A picture is produced by drawing pixels into the stencil store and using these pixels as an interpolation coefficient to control the combining of the two independent pictures.
Once a picture has been created to the user's satisfaction it can be printed. Printing of the picture normally involves the use of subtractive inks, namely cyan, magenta and yellow which are applied to a receiving medium in the form of dots which vary in size according to the intensity of a colour to be reflected from the medium. Areas of black in the picture can be printed by combining the cyan, magenta and yellow (CMY) inks but this can with some inks result in the black areas being seen as a brown colour rather than black in the printed picture. In order to overcome this it is common practice additionally to apply black (K) ink to the receiving medium.
In contrast to this, when a picture is displayed on a colour monitor the colour of each picture point is defined by the proportion of red, green and blue (RGB), i.e. additive colours, emitted from the display screen. Accordingly the system of the abovementioned patents is arranged to display pictures using data defining colours in terms of RGB components.
It is well known that in practice the relationship between the colours in a picture displayed on a monitor using RGB and the colours in a picture printed on a medium using CMY is non-linear and that there is a substantial range of colours which can be displayed on a monitor but which cannot be reproduced satisfactorily using CMY printing inks. Indeed, our own European patent application published as EP-A-0,245,943, the teachings of which are incorporated herein by reference, discloses a system comprising a colour conversion circuit for converting between RGB and CMY format for use at different parts in the system. The conversion circuit comprises a matrix of arithmetic circuits including look up tables which compensate for imperfections etc. in the printing inks when calculating the conversion between RGB and CMY.
Pictures to be modified can be input to our electronic graphic systems from any of a number of suitable sources. For example one source which is commonly used is a graphic art scanning machine which scans, say, a photograph and derives colour pixel data therefrom for delivery to another device, e.g. an electronic graphic system or a printer, for further use. Most scanning machines, and indeed many other sources in common use in the field of graphic art printing, present pixel data in CMYK format and it is therefore necessary to be able to convert this data to RGB format before it is used in our electronic graphic systems. Once the image data has been modified using our electronic graphic systems it is converted back into CMYK format for printing.
FIG. 1 of the accompanying drawings shows in schematic block diagram form the configuration that we have hitherto used in our electronic graphic systems to enable the user to make modifications to an original picture presented in CMYK format, before it is printed. In the system of FIG. 1 a source 1, which may for example be an image scanner or a disc storage device, delivers picture data as pixels in CMYK format to a conversion matrix circuit 2 which is arranged to convert the CMY data to corresponding RGB data. The conversion matrix circuit 2 may be of the kind described in our European Patent Application published as No. 245,943 the teachings of which form part of the present disclosure. It will be noted that the black (K) content of the pixel data from the source 1 is ignored by the converter circuit 2 and is therefore lost.
RGB data output from the conversion matrix circuit 2 is stored in a first colour framestore 3 for manipulation by the user. In the following description framestores will mainly be referred to either as monochrome framestores or as colour framestores. In the system shown in FIG. 1, and in the system according to the present invention to be described in greater detail hereinafter, colour pictures are defined using three bytes one each for the R, G and B, or C, M and Y, components. In the case of control image data or single colour separation data only one (monochrome) byte is required to define the data. Thus, a colour framestore is one having the capacity to store three bytes of data for each pixel whereas a monochrome framestore is one having the capacity to store a single byte for each pixel. The system configuration of FIG. 1 also includes a stylus/touch tablet device 6 by which the artist may modify the displayed image shown on the monitor 5. As the stylus is drawn across the tablet by the artist a number of signals are output from the device. Signals XY representative of the instantaneous position of the stylus on the tablet are output to a patch address generator 7. The patch address generator 7 converts the XY co-ordinate information from the tablet into a corresponding location, i.e. picture point, address in a framestore 8 and defines a patch of pixels about that location. It will be appreciated from a reading of our abovementioned British and U.S. Pat. Nos. that the framestore 8 may be used to store a colour image drawn by the user and that under these circumstances the framestore 8 will be a colour framestore, such as colour framestore 3. However, for the sake of clarity in this example it will be assumed that the framestore 8 is to be used to store a control image or stencil drawn by the user and accordingly that the framestore 8 is a monochrome framestore. Notional drawing implements are used to draw images into the monochrome framestore 8 and the system may be arranged such that after each update of the framestore 8 incremental movements of the stylus over the touch tablet are integrated until they exceed one picture point or similar spacing and then the framestore is again updated by stamping a modified patch of pixels in the framestore 8. A signal representing the instantaneous pressure of the stylus on the touch tablet is also delivered to a stylus pressure register 9.
A set of artist selectable notional drawing implements are each stored in a brush shape memory 10 as a numerical representation of a continuous three dimensional shape which covers a patch of image pixels. The address signal output from the patch address generator 7 is also used to synchronise addressing of the stylus pressure register 9 and the brush shape memory 10. Selection means (not shown in FIG. 1) are provided to allow the artist to select one of the drawing implements from the set.
In use data output from the stylus pressure register 9 and the brush shape memory 10 as the artist moves the stylus across the touch table are multiplied together to produce a coefficient for use by a brush processor circuit 11.
The brush processor circuit 11 performs a continuously cycling read-modify-write operation on the image data in the stencil framestore 8 on a pixel-by-pixel basis. Image data is extracted from the framestore 8 and is negatively summed with, i.e. subtracted from, a preset image intensity value held in an intensity register 12 by a summing unit 13 (.e.g. a 74S381 device). The resulting sum output from the summing unit 13 is multiplied with the interpolation coefficient by way of a multiplying unit 14 (e.g. a MPY-8HuJ/TRW device) and the resulting product is then added to the data extracted from the framestore 8 by way of an adding unit 15 (e.g. a 74S38i device). The data output from the adding unit is then written back into the framestore 8, replacing the original data in the framestore 8.
Although the read-modify-write process is executed in a continuous cycle on the data held in the framestore, it should be apparent that the data will only be modified when the artist is drawing on the touch tablet with the stylus. When the artist is not using the stylus/touch tablet device under pressure the data in the framestore 8 will remain unaltered and no read-modify-write cycles are performed. This method of processing data drawn into the framestore 8 avoids the problem of jagged edges by producing non-stepped boundary profiles.
The patch address generator 7, the stylus pressure register 9, the brush shape memory 10, the brush processor 11, and the intensity register 12 together form circuitry which will hereinafter be referred to as "drawing circuitry 16".
As has already been noted above, the drawing circuitry 16 can be used to modify directly RGB picture data in the colour framestore 3 by arranging for the drawing circuitry 16 to be connected to the colour framestore 3 instead of to the monochrome framestore 8. However, in this example, it will be assumed that the user wishes to combine data relating to another image with the data in the colour framestore 3 by way of a control image drawn into the monochrome framestore 8.
The control image in the monochrome framestore 8 is used together with a processor 17 to modify the RGB picture data in the first colour framestore 3. With the system configured in a drawing mode, RGB picture data from the first framestore 3 is output to the processor 17 together with other image data. The image data may be other RGB picture data delivered from a bulk storage device 18 and stored in a second framestore 19. The bulk storage device 18 may for example be a multidisc store adapted to store picture data. The processor 17 is arranged to interpolate the original RGB picture data from the first framestore 3 with that from the framestore 19, such that the data from each are selectively combined on a pixel-by-pixel basis in accordance with the control image data in the monochrome framestore 8. The control image data is used as an interpolation coefficient 1 to determine the contribution from the original RGB picture data and from the other RGB data. The thus modified pixels output from the processor 17 are written back into the first framestore 3 and in this way the original RGB picture data is modified. Modified pixel data output from the processor 17 is also written to a view framestore 20 and from there is delivered to a monitor 21 for display, thereby to enable the user to view modifications as they are made.
Once a modified picture satisfactory to the user, as displayed on the monitor 21, has been achieved, the modified RGB picture data is output from the first framestore 3 to a conversion matrix circuit 22 which converts each pixel from data in RGB format to corresponding data in CMY format. The conversion matrix circuit 22 performs the opposite function to that of the conversion matrix circuit 2. The CMY data output from the conversion matrix circuit 22 is input to a matrix circuit 23 which is arranged to derive the black (K) separation which is required for printing. The black (K) separation can be calculated in a number of methods each of which are per se well known. For example, one method which may be used in the matrix circuit 23 is to derive a so-called "skeleton black" as the black separation. Each colour component of the CMY data will be represented as a certain level of intensity below or equal to the maximum intensity for that component. To calculate the "skeleton black" the colour component with the lowest intensity is first identified. The intensity value of that colour component is then used to identify a point on a look up table and the corresponding "skeleton black" value is determined therefrom.
Another method which may be used in the matrix circuit is the so called "grey component replacement GCR method. The GCR method starts from the basis that the greyness (black contribution) of an image is determined by approximately equal contributions from each of the CMY components. In practice the cyan component is usually weighted more heavily than the magenta and yellow components to compensate for inadequacies in the printed cyan ink. The component with the lowest intensity value is identified and the value of intensity of that component is deemed to be the value for which equal contribution to the greyness from each component are made. The values are then used to identify a point on a look up table and the grey component is determined therefrom. The intensity values of the colour components is also adjusted by for example subtracting the equal contribution value from each of them to compensate for the extra, i.e. black, ink that will be printed in the printed image. Any other suitable known method of deriving the black (K) separation may instead be used in the matrix circuit 23, but the above mentioned "skeletonblack" method is preferred because it does not require any modifications to be made to the CMY data.
The black (K) separation and the CMY data representing respectively the cyan, magenta and yellow separations are delivered to a further storage means or directly to a printer (neither of which are shown) for storage or printing of the image as required.
The above discussed system configuration shown in FIG. 1 in most circumstances provides data which can be used to print a colour picture which will be satisfactory to the user. However, errors can be introduced when a picture is converted from CMY to RGB and back to CMY because there are many different ways in which the black separation may be defined. It would be difficult to store all methods and/or variations of parameters associated with each method in order to cover all eventualities.
Other errors can be introduced by the very nature of the picture. For example, in an image in which adjacent areas have complementary colours, for example red and cyan, the boundary between these areas will be clearly visible as a grey line. One reason for this is because our electronic graphic systems define changes between colour areas as a gradually changing curve. The curve starts with one colour and ends with the other and between these two colours includes increasing and decreasing contributions from each. When the two colours are complementary, the effect of adding them together will be to cancel out the colour so that only a black component, i.e. no colour, remains.
Furthermore, when the RGB version of the picture data is modified, the black separation will also be changed and it is not possible simply to store the original black for later use in printing the modified image. Nor is it possible simply to replace the original black component in the modified areas with the black component of the modified area because the black component of the modified area will almost certainly be derived in a different manner to that of the black component of the original image and this will make any such modifications visible in the final image.
The invention resides in the realization that there is a need for a system in which black separation data is made available for modification independently from the remaining data representing the image. In this way a user would be able to make modifications only to the black separation or to a black separation derived from data relating to a modified image or images prior to printing the image.
There is also a need for a system in which a picture originally represented by pixel data in CYMK format, converted to RGB format to effect changes to the picture and then converted back to CMYK format for printing can be further edited to modify features or remove unwanted features from the black (K) separation before printing.
Furthermore, it would be desirable to provide a system in which an operator controlled arrangement is provided to enable selective modification of data representing a black separation.
In one aspect the present invention provides an electronic graphic system for use in modifying original black and colour separation data defining an image, in which system the black separation data is stored whilst the original colour separation data is modified to produce modified colour separation data and, once desired modifications to the colour separation data have been achieved, black separation data is derived from the modified colour separation data, and wherein the original black separation data and/or the derived black separation data can be selectively modified by the user to produce modified black separation data which is output together with the modified colour separation data for use in for example printing a modified image.
In another aspect the present invention provides an electronic graphic system for use in modifying black and colour separation data defining an image, the system comprising, first monochrome storing means for storing the black separation data, first colour storing means for storing the colour separation data, modifying means for selectively modifying the colour separation data and means for deriving therefrom derived black separation data to be stored in a second monochrome store means, said modifying means being adapted to enable the original black separation data and/or the derived black separation data also to be modified, and means for outputting the modified black separation data and the modified colour separation data.
In a further aspect of the invention there is provided an electronic graphic system comprising image processing means for creating or modifying an initial image represented by colour separation pixel data without black separation date, means for deriving black separation pixel data pertaining to the image created or modified by said image processing means, and operator controlled modifying means for selectively modifying said derived black separation data.
The invention in another aspect provides an electronic graphic system for use in modifying image data prior to printing the image represented by the data, in which system data relating to the black separation of the image is made available for modification independently from the remaining data representing the image.
The above and further features and advantages of the invention will become clearer from consideration of the following detailed description of an exemplary embodiment of the invention given with reference to the accompanying drawings.